Evolution of the semiconductor manufacturing industry is placing ever greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions are shrinking while wafer size is increasing. Economics is driving the industry to decrease the time for achieving high-yield, high-value production. Thus, minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for the semiconductor manufacturer.
Fabricating semiconductor devices, such as logic and memory devices, typically includes processing a semiconductor wafer using a large number of fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a photoresist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection is used to find defects in semiconductor devices on a wafer as well as defects on blank, unpatterned wafers. False positives, or false counts, are undesirable in any inspection situation. False counts in an inspection system can arise from multiple sources. This may include electronic noise associated with detectors in the system as well as external noise associated with photons or radiative particles from sources other than the sample of interest. In the context of inspection systems, a false count occurs when a signal not associated with a sample is detected by one or more detectors and is incorrectly associated with properties of the sample.
Unwanted radiation gives signal from a time delay and integration (TDI) sensor over an entire area that has vertical clocking voltages applied to it. This is the region where the collected photocharge makes up a moving photoelectron image that is synced with the optical image as a wafer is scanned. Both stray light and air scattered deep ultraviolet (DUV) light have a broader footprint in the image plane than the illumination profile that provides the desired signal from defects on the wafer. Illuminated air above the wafer plane, such as at distances from 50-200 μm, can cause scattering. Air scattered light and scattered stray light tend to originate in planes far from the image plane in the z direction, which is at right angles to the imaging plane. Such points are not brought to a focus in the imaging plane so they tend to be spread out.
Therefore, a technique to reduce unwanted stray and air scattered light is needed.